Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
2002-08-06
2004-10-12
Denion, Thomas (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C123S090150, C123S090600, C074S569000
Reexamination Certificate
active
06802287
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a valve mechanism, in particular for internal combustion engines of motor vehicles, having at least one driven cam element and having a valve actuator, which can be displaced or pivoted by the cam element, the cam element being arranged rotatably in a flexible surround element which is connected to the valve actuator in such a manner that it can move in a plane which is perpendicular to the axis of rotation of the cam element.
Valve mechanisms for controlling the valves of internal combustion engines, in particular for motor vehicles, usually have a device (spring, hydraulic element, etc.) which is used to load the valve toward its closed position. In this position, a valve actuator (valve lifter, drag lever, rocker lever or the like) is pressed against a continuous valve control surface, which in part runs eccentrically with respect to the shaft axis. When the valve is closing, it should be ensured that the valve disk does not strike the valve seat too quickly, since otherwise it will rebound. This requires relatively complex matching between the shape of the cam, the masses which are to be moved, the forces which are generated, the materials properties, etc.
Therefore, there is no lack of proposals concerning forced guidance of the valve actuator on the cam element; various embodiments have been developed, which are each based on two eccentric valve control surfaces instead of the restoring spring. Specific designs are to be found, for example, in British patent specifications GB 19,193(1913) and GB 434,247, wherein the cam element, on at least one end face, has a groove, the two side walls of which form the valve control surfaces. A roller or the like which is arranged at the end of the valve actuator engages in the groove from the side. A cam element which has a web which can be gripped around is known, for example, from European publication EP 429 277 A.
A further proposal for a desmodromic valve mechanism, wherein a space-saving, lightweight and inexpensive design is achieved, is to be found in published German patent application DE 37 00 715 A which describes the generic type referred to in the introduction. In this design, a surround element is provided, which surrounds the circumference of the cam element without significant play, so that it always matches the shape of the cam, yet the cam element, on account of the nature of the surround element, can rotate inside the latter. Since the surround element connected to the valve actuator cannot rotate with the cam element, the movement of the cam region about the axis of rotation of the cam element is converted into a lifting or reciprocating movement of the valve actuator which is mounted displaceably or pivotably in the cylinder head. The valve actuator does not execute a movement as long as the connecting region of the surround element together with the valve actuator rests against the base circle region of the rotating cam element, is then moved away from the axis of rotation of the cam element in the radial direction and finally is returned again, while the cam region of the cam element moves past the connecting region of the surround element and the valve actuator. The moveable connection of the surround element to the valve actuator allows the pivoting or tilting movement of the surround element in the cam region, so that the required freedom of movement of the valve actuator in its sliding or pivot bearing is preserved. In the first exemplary embodiment, the surround element is formed by two flexible rings, between which needle-shaped rolling bodies are provided in order to reduce the friction. A second embodiment provides a plastic strip having an inner ceramic slip layer.
Particularly when the valve mechanism is used in internal combustion engines, a surround element is subject to high loads, and it is necessary to rule out temperature- or fatigue-related plastic lengthening of the surround element. An irreversible increase in the size of the gap between the circumference of the cam element and the surround element affects in particular the valve-closing position.
Furthermore, the term variable valve control has revealed a wide range of different structures which can be used to change the opening and closing time and the lift of the valve, in order to improve the performance, the exhaust emissions, the torque, etc. of an internal combustion engine. Compared to the non-adjustable valve control with fixed values, the filling of a cylinder is improved if the valve is opened later and closed earlier at low rotational speeds and is opened earlier and closed later at higher rotational speeds. It is therefore possible, by means of a speed-dependent adjustment of the valve control, to optimize the exhaust emissions, the torque, the engine performance, etc. All the variable valve control arrangements which have been revealed to date change the position of the actuating surface of the valve actuator relative to the eccentric valve control surface through rotation, linear displacement or enlargement of the cam element. These adjustment mechanisms are relatively complex and, in some cases, also require considerable adjustment forces, since they have to operate counter to the restoring elements of the valves.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a valve lifting mechanism, particularly for an internal-combustion engine, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a variable forced valve control.
With the foregoing and other objects in view there is provided, in accordance with the invention, a valve mechanism, comprising:
at least one driven cam element and a valve actuator driven by said cam element;
a flexible surround element, said cam element being rotatably disposed in said flexible surround element about an axis of rotation and said flexible surround element being movably connected to said valve actuator for movement in a plane perpendicular to said axis of rotation of said cam element, and wherein said surround element is configured to be reversibly lengthened for adjusting a valve lift of said valve actuator.
In other words, the objects of the invention are achieved, in a valve mechanism of the type described in the introduction, in that the surround element is designed so that it can be lengthened reversibly.
The cam element rotating in the surround element generates tensile forces, which rise as a function of rotational speed, at the connecting point to the valve actuator, so that the surround element, which bears against the circumference of the cam element virtually without play at idling speed, is lifted increasingly further away from the circumference as the rotational speed rises, thus adopting positions which correspond to cam elements with greater circumferential lengths. Since, in this way, the distance between the axis of rotation of the cam element and the connecting point between the surround element and the valve actuator increases, an additional valve lift which is dependent on rotational speed is produced.
In a first embodiment, the reversible lengthening of the surround element is achieved by elastic stretchability of at least a partial region of the surround element, so that the play which is formed between the cam circumference and the surround element is reduced further as the rotational speed falls. Moreover, it allows advantageous, slight prestressing of the surround element in the at-rest state, in order to ensure that the valve-closing position is reached outside the cam region despite any temperature-related changes in length.
The surround element may consist of an elastically stretchable material or may be composed of two materials with different properties, at least one of which can stretch elastically. By way of example, a non-stretch strip may be closed with respect to the surround element by an elastically stretchable intermediate piece, wherein case a holder for the valve
Corrigan Jaime
Denion Thomas
Greenberg Laurence A.
Locher Ralph E.
Stemer Werner H.
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